Storage systems in which a plurality of storage apparatuses redundantly store the same data are being developed as storage systems for storing data in order to avoid loss of stored data. For example, when a user accesses a specific server through the Internet, and writes data, the written data is stored in a certain storage apparatus, and further, the same data (replication data) as the data stored in the storage apparatus is also stored in another storage apparatus. In this manner, by a plurality of storage apparatuses holding the same data, even if it is not possible to read data from one of the storage apparatuses, because the data is mistakenly deleted in the one of the storage apparatuses, or a failure occurs in the one of the storage apparatuses, it is possible to read the data by accessing the other storage apparatuses. Holding the same data in a plurality of storage apparatus is called replication.
It is thought that replication is achieved by a method in which first, one storage apparatus writes data, further, that storage apparatus transmits the replication data of that data to another storage apparatus, and the other storage apparatus stores the replication data. In this case, if communication congestion occurs in a network coupling two storage apparatuses, data transfer between the storage apparatuses sometimes takes long time. Also, if transmission is performed individually for a plurality of pieces of replication data, the number of data transmissions increases. This causes communication congestion in the network, and further causes a delay in the data transfer. Concerning this problem, it is known that there is a method in which a plurality of pieces of data are put together, and are transmitted between the storage apparatuses so that the number of communications is reduced. As a result of using this method, the processor load of the storage apparatus for performing communication processing is reduced, the communication load of the network is reduced, and thus time occupied for storing replication data is also shortened. Japanese Laid-open Patent Publication No. 2004-133598 is given as one of related-art technical documents.